The combination of minimal tool exterior body area and demand for increased maximum torque/power deliverance sets high demand on the angle gear. In tools of the above type, there is always a problem to obtain an efficient enough cooling of the gearing or angle gear, due to heat generated during operation of the tool. Excessive heat in the gearbox is not allowed for large continuous rating. Problems that normally can occur are related to oil leakage due to excessive heat combined with wear of seals of the outgoing tool shaft. Especially tools with an outgoing shaft rpm exceeding 12000 rpm, having a very high periphery seal contact speed; it is found that both angle gear and seals are overheated in applications where the tool is continuously used. The primary problem to be solved is to minimise the temperature of the angle gearing during operation of the power tool. A secondary problem is to seal the angle gear and improve lubrication possibilities by minimising lubricant leakage due to wear of seals at the outgoing tool shaft.
A previously known way of solving heat problems is to use the cold exhaust air from the air motor to transport heat from for instance an impulse generator to the outside of the tool housing. An example on that is illustrated in U.S. Pat. No. 4,418,764. The tool shown in this patent is of the pistol handle type in which the housing is formed with an exhaust air passage that extends from the motor, past the impulse generator and out into the atmosphere via outlet openings at the forward end of the tool housing. The exhaust passage extends from a number of outlet openings on the motor cylinder and through cavities formed in the housing it has been easy just to design the casting of the housing to comprise the space necessary to accomplish a desired exhaust air flow.
Another known way of solving the heat problem is disclosed in U.S. Pat. No. 5,626,198, where a pneumatically powered torque impulse delivering tool for screw joint tightening comprises a housing with a forward impulse chamber enclosing a hydraulic impulse generator with air inlet and outlet passages located at the rear end of the housing. The motor cylinder is provided with radial air communication openings and outer grooves forming passages for connecting the openings to the air inlet and outlet passages. The exhaust air leaving the motor is ducted to the air outlet via rearward extending grooves, whereas part of the outlet air is communicated into the impulse chamber via forwardly extending grooves. From there on the exhaust air is ducted to the rear end of the motor and to the rear outlet passage. During its circulation through the impulse chamber, the cold exhaust air absorbs heat from the impulse generator and transports that heat out of the tool. With these types of machines circulation around the impulse chamber is easily obtained due to the fact that impulse chambers are tightly sealed. The impulse generator has its own internal lubrication and there is no desire for a forward exhaust outlet.
In the type of tools comprising an angle gear, however, i.e. angle grinders, the outer diameter of the tool housing has to be kept relatively small to offer a comfortable grip for the operator. When in such tools it also becomes desirable to arrange cooling air flow not only to and from openings in the motor cylinder to an exhaust passage at the rear end of the tool, but also past the angle gear and to an exhaust passage at the forward end of the tool. The problem is to obtain passages that do not interfere or enter the gear chamber but still have sufficient cooling effect.